Lapping machines are used to treat the surfaces of workpieces using an abrasive assembly. Typically, lapping machines include upper and lower lapping discs which are made of a strong material and have precisely flat working surfaces which are loaded with an abrasive compound. Workpieces are carried within a cassette that is positioned between the upper and lower lapping discs and which effect treatment of the workpieces. Typically, the axes of the lapping discs are vertical and coaxial and treatment is accomplished by rotating one or both discs relative to the cassette such that the appropriate surfaces of the individual workpieces are abrased by the working surfaces of the lapping discs.
During the treatment of workpieces, it is desirable to ensure minimal differences in thickness (or diameter if the workpieces are cylindrical) between individual workpieces and that the top and bottom sides of each workpiece are essentially parallel to eachother and flat. Care must be taken to avoid lapping constantly over the same areas of the lapping disc so that the surface of lapping disc is evenly utilized. If the working surfaces of the lapping discs are unevenly worn, the surfaces of the workpieces cannot be lapped to a high finish and to an accurate flatness. Since the speed of each particle on the abrasive surface of the lapping discs is proportional to the distance between the centre of rotation and the particle, it is necessary to provide workpieces with an appropriate trajectory of movement between the lapping discs in order to minimize radial deformation of the working surfaces of the lapping discs and to optimize the accuracy of finishing of the workpieces.
In order to ensure even wear of the working surfaces of lapping discs, prior art lapping machines utilize cassettes that have sockets formed within for holding individual workpieces and which allow the cassette to be rotated independently from the lapping discs. Accordingly, the workpieces are provided with radial movement having a changing vector of velocity relative to the vertical axis of the lapping discs and move along a spiral trajectory relatively to the lapping disks.
One type of assembly which effects such movement of workpieces is a planetary lapping machine as in U.S. Pat. No. 3,662,498 to Caspers and U.S. Pat. No. 4,157,637 to Orlov et al. A planetary lapping machine utilizes a number of circular cages, each cage having individual workpiece sockets. Each cage is driven by a sun gear and a ring gear of a sun-and-planet gear which are rotated on vertical shafts coaxial with annular upper and lower lapping discs. This construction ensures that the center of workpieces being machined are moved along spiral trajectories over the working surface of the lower and upper lapping discs. Planetary movement is composed of the joint motion of the circular motion of each individual workpiece socket rotating within a circular cage around the individual cage axis and the larger circular orbiting movement of each socket around the axis of the lapping discs.
Another type of lapping assembly is disclosed in U.S. Pat. No. 3,541,734 to Clar, which describes a dual-disc lapping machine. A dual-disc machine is generally used for lapping cylinders and includes a cassette which is driven by an eccentric crank pin so that the cassette rotates in an eccentric manner and at a different speed than the upper and lower discs.
Both types of lapping assembly suffer from substantial and well known disadvantages. Lapping discs are unevenly worn during the course of treatment of workpieces. Also, the required spacing of individual workpieces inside the cassettes results in a relatively low yield of treated workpieces from the lapping machines. Finally, since each workpiece must be individually housed within cassette sockets, labour intensive loading and unloading of workpieces from the cassette is required.
These disadvantages can be overcome as described in USSR Patent No. 181,516-66 to Malkin, by vibrating one or both of the lapping discs using a vibration generator to provide translational circular motion therein. Further, the lapping discs can be provided with additional motion so that the axis of the lapping discs circumscribe a cone. This techniques improves the precision and productivity of the workpiece finishing process by allowing loosely packed workpieces to move between the upper and lower lapping discs in an orderly manner.
Further, as described in USSR Patent No. 227,127-67 to Malkin, one of the lapping discs can be suspended on a shock-absorber and fixedly secured to an unbalanced-mass vibration generator. The lapping discs are connected to each other by elastic elements such as, for example, radial helical coil springs, whose total rigidity exceeds the total rigidity of the shock-absorber. This apparatus provides simultaneous finishing of both surfaces of the workpieces and allows the user to adjust the amount of material to be removed from the workpieces.
One disadvantage of these approaches is that particles on the surface of the lapping disks move in a vertical direction at a linear acceleration that is proportional to the radial distance of the particle from the center of the lapping discs. This causes the surfaces of the lapping discs to be worn into a conjugate spherical shape. This defect is especially apparent when large workpieces are treated and accordingly, such an approach is most appropriate for small workpieces. Also, prior art machines do not effectively move individual workpieces in relation to particles on the surface of the lapping discs, especially when the pressure applied to the surfaces of the workpieces is substantial. Finally, it is difficult to apply the same level of treatment to both sides of a workpiece, since the intensity of oscillations applied to one lapping disk connected to vibrator is higher than that of the other disk (especially when the working pressure is high) and since oscillations are imperfectly transferred through elastic couplings. This results in workpieces being treated primarily on one side.
Accordingly, there is a need for an improved lapping assembly which provides even treatment to both surfaces of a group of workpieces, which effects even wear to the surfaces of lapping discs, which is easy to load and unload, which allows for treatment of a large number of workpieces, which comprises relatively few parts, and which is durable and relatively inexpensive to manufacture.